Projectile X-ray detection: application and limits

Nuclear

Instruments and Methods in Physics Research B 99 (1995) 519-523

g

Beam Interactions with Materials&Atoms

!&J ELSEVIER

Projectile X-ray detection: application and limits Michael J.M. Wagner a,

*,

Hans-Arno Synal b, Martin Suter a, Jean-Luc Debrun ’

aInstitut fir Teilchenphysik, Eidgendssische Technische Hochschuie, CH-8093 Ziirich, Switzerland b Paul Scherrer Institut, c/o ETH Ziirich, CH-8093 Ziirich, Switzerland ’ Centre National de la Recherche Scientijique, CERI, F-45071 Orleans, France Abstract Projectile X-ray detection (PXD) has been studied to estimate the potential of this ion identification technique in accelerator mass spectrometry CAMS). Limits and possible applications of PXD are discussed and compared to standard isobar identification techniques with gas ionisation chambers. The isobar discrimination capability of a gas ionisation chamber depends on mass, nuclear charge and energy of the ions to be identified. At about 1.5 MeV/amu, suppression factors higher than lo4 and efficiencies close to 1 have been achieved for ions of masses up to 40 amu. The isobar discrimination capability of PXD is nearly independent of the ion energy, and suppression factors of 10 to 100 are possible. The detection efficiency depends on the ion-target combination, the detector arrangement and the energy of the projectiles. Our results show, that PXD can compete only at lower energies or higher masses, for which gas counters give insufficient isobar discrimination. For the PXD method detection limits on the order of 10-r* for 36 Cl/Cl, lo-* for 59Ni/Ni and lop7 for 126Sn/Sn were reached.

1. Introduction

comparison of the detection limits of this technique gas ionisation chambers is made.

In accelerator mass spectrometry CAMS) measurements of long lived radioisotopes such as “Be, 36C1, 59Ni, 12?Sn the interference of the stable isobars ‘“B, 36S, 59Co, lz6Te is, in many cases, the limiting factor. In the past primarily gas ionisation chambers have been used for the final identification. In ionisation chambers the Z-dependence of the stopping power is used to discriminate the interfering isobar. In gas-filled magnets, another isobar separation tool, the Z-dependence of the mean charge of the ions travelling through a low pressure gas is utilised to suppress the isobars. The separating power of these techniques depends strongly on energy and atomic number. The discrimination of the heavier isobaric pairs (36C1/36S, 5YNi/ 59Co, etc.) requires relatively large accelerators. Therefore other isobar separation techniques are required at lower energies. Projectile X-ray detection (PXD) was recently presented as a new heavy ion detection technique [l-3]. In PXD the characteristic X-rays emitted by the ions stopped in a target foil are measured. By the appropriate choice of the target material, molecular excitation processes can be utilised to maximise the X-ray yield [4]. In this paper X-ray yield data and suppression factors for 36C1, s9Ni, “‘Sn are presented. Based on these data a

with

2. The detection techniques 2.1. Gas ionisation chambers The set-up for the use of gas ionisation chambers in AMS has been described elsewhere [5,6]. The isobar suppression power depends mainly on the spacing of the two isobar peaks in the energy loss spectrum (S(AE)), on their width (a) and their shape. The split anode detector can be optimised by adjusting gas pressure and electrode length in such a way that 6(AE) is maximal [7]. The simple model described in Ref. [7] uses 6(AE) and LY,calculated from energy loss [8] and straggling data [9] and a Gaussian shape of the energy loss distribution. The tails of the interfering isobar evaluated in the region, where the signal of the radioisotope is expected, are integrated to calculate suppression factors. The values in Fig. 1 are calculated for _t 1 o integration limits (68% integration). The point symbols represent experimental data. 2.2. Projectile X-ray detection

*Corresponding author. Tel. +41 1 633 2026, fax +41 1 371 2665. e-mail [email protected]. 0168-583X/95/$09.50 0 1995 Elsevier SSDI 0168-583X(95)00046-1

The experimental set-up is described in Ref. [2]. The X-ray detector is positioned directly behind the target foil to maximise the solid angle. The detector used for these

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M.J.M. Wagner et al. /Nucl. Instr. and Meth. in Phys. Res. B 99 (1995) 519-523

520 ,.OE+6

,.m+5

I

c

,.OE+O *.iX+,

“‘I

Ni

%

/R

I

IIII

1

LOS+8 E [MeV]

Fig. 1. Isobar suppression factors of PXD and gas ionisation ),Ni(------)andSn(.-.-.).Thehoridetectors: Cl (zontal lines are for PXD based on experimental values and under the assumption of energy independence. The factors for gas detectors are calculated for + l(+ peak integration [7]. Experimental values for Cl (0) are shown.

has a 28 mm* active area and covers a solid angle of about 3%. For 36Cl and ‘26Sn measurements a 6 pm Ti foil was used, for 59Ni 10 p,m Zn foil in combination with a 10 pm Co absorber. The Ti foil for Cl and the Zn foil for Ni were selected, because of the enhanced production cross section for K-vacancies for near symmetric collisions [4]. In the case of Sn, a Ti target provides high L-vacancy production due to the matching of the Ti K and Sn L orbitals [lo]. experiments

channels

Fig. 2. X-ray spectra of (a) 36Cl blank measurement on a Ti target foil, (b) “Ni blank on a Zn foil with Co absorber, (c) tz6Sn blank on a Ti foil, (d) 5gNi blank on a Zn foil without Co absorber with + 1 D integration limits. The position of the radioisotope was determined by using standard samples.

the X-ray transmissions. To calculate the X-ray absorption coefficients the energy of the shifted K,/L,-lines are used: s=--

NI TR

NR

2.2.1. Suppression factor

The isobar suppression power depends on the line structure of the X-ray spectrum. This structure is influenced by energy shift and splitting of the X-ray lines due to the high degree of ionisation, the Doppler shift, and by the difference of the yields of projectile and target X-rays. These effects cause a slight energy dependence. The suppression power can be optimised by an appropriate foil combination. When the X-ray line of the radioisotope is interfered with by a line of higher energy, this line can be reduced by an absorber with the absorption edge between these two energies. For the determination of the suppression factors, the blank sample measurements, which show only the lines of the interfering isobars (Fig. 2) are used. The region of the radioisotope has been determined in previous measurements of standard samples. The ratio of the isobaric counts (N,) to the Bcquired counts in the region where the radioisotope is expected (N,) must be corrected by the corresponding detection efficiencies. They depend on the target yield, the X-ray transmission through the target and the detector entrance foil, and on the solid angle of the detector (see Section 2.2.2). The target yield and solid angle are assumed to be equal for the radioisotope and the isobar. Therefore N/N, must be multiplied by the ratio of

TI '

where s is the suppression factor, N,/N, is the count ratio from Fig. 2, TR is the X-ray transmission of the radioisotope, and T is the X-ray transmission of the isobar. The values for 36Cl, 59Ni and 126Sn are listed in Table 1. 2.2.2. Detection efficiency According to Meyerhof et al. [4] the thick target X-ray yield follows a Y-E p law. p is constant over a wide energy range and is slightly increasing with Z. Using the data from nickel and neodymium in the energy range from 15 to 21 MeV [3] p can be estimated ( p = 3). In Fig. 3 solid lines with this slope are drawn though the available data points from [3] (A) and data from this work (X ). This data was obtained by measurements on thick targets

Table 1 Values for the PXD suppression factors 36Cl 5gNi a ‘?Sn

&/NR

Tll

TI

s

13 76 17

27% 41% 53%

16% 32% 60%

18 97 15

a A 10 pm Co foil was used to absorb the Co Kg line. For more information see Section 4.2.

MJ.M.

Table 2 Parameters

for the calculation

PXD Gas detector

36cl

.,,,,*-o-

b

;

4

,’

-L--LLL

,.OE-5 1.LX+,

of the detection limits in Fig. 4

E

s

CR [Hz]

see Fig. 3 68%

see Table 1 see Fig. 1

0.03 0.03

CI

T

lo-”

10%

In.4

I,,

I.OE-4

521

Wagner et al. / Nucl. Ins@. and Meth. in Phys. Res. B 99 (1995) 519-523

10000

7x 10-7 3X10-h

100 10

59Ni ‘26Sn

5% 2%

‘.OE+2

E [Met’]

Fig. 3. X-ray target yields (solid lines) and detection efficiencies (dashed lines) for 3”C1, 59Ni and 12?n PXD measurements. The target yields are fitted to experimental data (A 131, X (this

work)), assuming the same energy dependence for each element. The detection efficiencies are calculated from the target yields taking into account solid angle and absorption. Experimental data (0) are shown for comparison.

the first step for isobar suppression is a chemical purification of the samples. For chlorine a typical treatment is described in Ref. [ll] for nickel in Ref. 1121. Typical isobar beam fractions are listed in Table 2. Assuming that a reliable detection of the radioisotope is only possible if its count rate is higher than the background rate, the detection limit (Lao) is given by: L *G=-.

c, S

with the X-ray detector in a 135” position. The measured K, (for Cl and Ni respectively) and L, (for Sn) count rate was corrected for the absorption in the detector entrance foil and the solid angle of the detector. This was determined by a measurement of a calibrated 55Fe source. To get the target yields this count rate was normalised to the beam current measured in a Faraday cup before and after the X-ray measurement. The detection efficiency for an Ah4S measurement depends on the X-ray yield of the projectile-target combination, the absorption in the target foil (the detector is positioned behind the foil) and the solid angle covered by the X-ray detector. The detection efficiencies (E, dashed lines in Fig. 3) are calculated from the target yields drawn in solid lines by

Second the required counts within a defined measuring time sets an additional limitation. To get 10% counting statistics within a 1 hour measurement, a minimal count rate of 0.03 Hz is required. This defines the efficiency detection limit L,: CRn, L,=

&Il_ET

CR is the minimal count rate, n, is the abundance of the stable isotope to which the measurement is normalised, E is the detection efficiency, I, is the current of the stable isotope extracted from the ion source, and T is the acceler-

,.OE-6

E = YOT,,

I OS-7

k

Sn

OS

i

‘.

,

-.

where Y is the target yield, R is the solid angle (3%), and TR is the X-ray transmission (Table 1). The experimental efficiencies (Fig. 3, 0) were determined in AMS measurements of samples with known isotopic ratios.

3. Detection limits In general, the detection limit of a radioisotope in an AMS measurement is determined by two factors. First there is the background level (Lno). It depends on the abundance of the isobar (c,) and on the suppression factor (s). This abundance is defined as the fraction of isobaric ions in the beam of interest. It depends on the chemical composition of the sample, the negative ion yields and on fractionation effects during the beam transport. Therefore

E [Met’]

Fig. 4. Background level (Lao) and efficiency detection limit CL,) for 36C1, 59Ni and 126Sn measurements. The results for gas detectors (.-.-. (La,), ..-..-.. CL,)) and PXD ((Lao), --CL,)) are shown. Experimental data: 0. Upper and bottom limit of the values l-6 show Lao and L, according Table 3.

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ator transmission. In Fig. 4 the detection limits L,, and L, for 36C1,5gNi and ‘%‘n for gas counters and PXD are shown. For gas detectors s has been taken from Fig. 1. The efficiency for PXD has been taken from Fig. 3. The other parameters can be seen in Table 2. The low energy currents (Z,) are typical values for a gun type Cs sputter source [13]. The use of a high current ion source can increase these currents by factors of 3 to 10 [14]. For the accelerator transmission (T) typical values from the 6 MV EN-tandem in Zurich are taken. The experimental data points are listed also in Table 3.

4. Possibilities for the application of PXD Fig. 5. Energy loss spectrum of a 59Nistandard (59Ni/Ni = 1.1X 10m6) at 72 MeV with asymmetricpeak integration.

4.1. 36cI

In the case of 36C1the present detection limit of PXD measurements is lo-“. This is not competitive with detection limits obtained with gas ionisation chambers in the accessible energy range of medium sized tandem accelerators. A significant improvement of the limit due to increased efficiency and currents could make PXD competitive in the energy range from 10 to 20 MeV. The use of a high current source, a special large-area windowless X-ray detector and a target foil arrangement with less self-absorption might improve the performance by more than one order of magnitude. This would still be insufficient for detection of natural 36C1concentrations in most cases. 4.2. “Ni The 5gNi PXD detection is limited by the background at the lo-* level. To get less background from tails of X-rays of higher energy than the Ni K, an absorber foil has been tested. Because the energy of the projectile X-rays is shifted to higher energy [2], the Co Kp can be attenuated in a 10 pm Co foil. Compared with Fig. 2d (Ni blank without absorber), Fig. 2b shows much less of the Co Ka and the Zn lines, but a shoulder occurs in the region were the 5gNi events are expected. This shoulder results from Ka fluorescence of the Co absorber foil. The use of a Ge target foil will reduce the yield of target

X-rays [4] and will therefore lower the fluorescence shoulder. This and a cleaner sample preparation will improve the measurement limit. But for the measurement of meteorites and other extra terrestrial samples a detection limit lower than 10-i’ would be necessary [12]. Therefore the application of PXD will be limited to the measurement of samples containing anthropogenic radioactivity. First measurements of nuclear waste samples were carried out in collaboration with Debrun et al., CNRS, Orleans. To get a detection limit below the 10m8 level at our AMS facility an experiment at 72 MeV (6 MV terminal voltage, ll+ charge state) with a gas detector was carried out. To reduce the count rate of molecular background in the detector, the analysed high energy beam was post stripped to the 18+ charge state and filtered by a 180” magnet. In the energy loss spectrum only half of the 5gNi peak was integrated (Fig. 5). With such cuts in every AE spectrum of the split anode detector a suppression factor of 3000 and a background level of 2 X lo-” could be reached (see data point 3 in Fig. 4). The disadvantage of this set-up is the high instability due to changes in the peak positions in the energy loss spectra of Co and Ni caused by high count rates. Other techniques used for isobar separation in “Ni measurements like gas-filled magnets and full-stripping are

Table 3 Values and references of the data points in Fig. 4 Point No. 1 2

36c1 36c1

3

“Ni

4 5 6

” Ni “Ni lz6Sn

Technique gas det. PXD

E [MeV]

gas det. gas det. PXD PXD

72 210 18 40

48 27

L, 7 x lo-‘5 8x10-‘*

LBG

1 x 10-9 4x lo-= 6 X 1O-9

3x10-‘0 5x1o-‘3 1 x 10-s 6X1O-7

1x10-‘5 3x 10-13

Ref. Synal et al. 1111 this work this work Paul et al. [12] this work this work

M.J.M. Wagner et al. /Nucl.

Ins@. and Meth. in Phys. Res. B 99 (1995) 519-523

reviewed in Paul [15]. 59Ni in meteorites could so far only be detected at energies above 200 MeV. 4.3. 126S?l “%n is a long-lived fission product. It might become interesting in investigating aspects of nuclear waste storage. PXD provides the unique possibility for ‘%n measurements with energies below 50 MeV. Experiments with ‘*%n are in progress in collaboration with the University of Vienna. First tests on a ‘%n sample material, which has been previously measured at Argonne National Laboratory with a gas-filled magnet and an ionisation chamber at an energy of 400 MeV [16], have shown, that identification of ‘*%n is possible with PXD at the 10m6 level.

5. Conclusion PXD opens the possibility for small AMS machines (terminal voltage < 3 MV) to measure radioisotopes like The detection 36C1 s9Ni and ‘*%n with Z-identification. limiis are above the natural concentrations of these isotopes. The application is therefore confined to samples with anthropogenic radioactivity like nuclear waste samples.

Acknowledgement This work was supported in part by the Swiss National Science Foundation.

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References [l] H. Artigalas, M.F. Barthe, J. Gomez, J.L. Debrun et al., Nucl. Instr. and Meth. B 79 (1993) 617. [2] M.J.M. Wagner, H.-A. Synal and M. Suter, Nucl. Instr. and Meth. B 89 (1994) 266. [3] H. Artigalas, J.L. Debrun, L. Kilius, X.L. Zhao, A.E. Litherland, J.L. Pinault, C. Fouillac and C.J. Maggiore, Nucl. Instr. and Meth. B 92 (1994) 227. [4] W.E. Meyerhof, R. Anholt, T.K. Saylor, SM. Lazarus and A. Little, Phys. Rev. A 14 (1976) 1653. [5] M. Suter, R. Balzer, G. Bonani, M. Nessi, Ch. Stoller and W. Walfli, IEEE Trans. Nucl. Sci. NS-30 (1983) 1528. [6] H.-A. Synal, J. Beer, G. Bonani, H.J. Hofmann, M. Suter and W. WGlfli, Nucl. Instr. and Meth. B 29 (1987) 146. [7] M. Suter, Nucl. Instr. and Meth. B 52 (1990) 211. [8] L.C. Northcliffe and R.F. Schilling, Nucl. Data Tables A 7 (1970) 233. [9] H. Schmidt-BGcking and H. Hornung, Z. Phys. A 286 (1978) 253. [lo] W.E. Meyerhof, A. Riietschi, Ch. Stoller, M. Stiickli and W. WSlfli, Phys. Rev. A 20 (1979) 154. [ll] H.-A. Synal, J. Beer, G. Bonani, Ch. Lukasczyk and M. Suter, Nucl. Instr. and Meth. B 92 (1994) 79. [12] M. Paul, L.K. Fifield, D. Fink, A. Albrecht, G.L. Allan, G. Herzog and C. Tuniz, Nucl. Instr. and Meth. B 83 (1993) 275. [13] R. Balzer, G. Bonani, M. Nessi, Ch. Stoller, M. Suter and W. Wiilfli, Nucl. Instr. and Meth. B 5 (1984) 204. [14] Th.R. Niklaus, F. Ames, G. Bonani, M. Suter and H.-A. Synal, Instr. and Meth. B 92 (1994) 96. [15] M. Paul, Nucl. Instr. and Meth. A 328 (1993) 330. [16] W. Kutschera, I. Ahmad, P.J. Billquist, B.G. Glagola, R.C. Pardo, M. Paul, K.E. Rehm and J.L. Yntema, Nucl. Instr. and Meth. B 42 (1989) 101.

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